nixos/h2o: enable HTTP/3 via QUIC

nixos/modules/services/web-servers/h2o/default.nix

···
                             headerSet ++ [ hsts ];
                       }
                     );
-              in
-              value.settings
-              // headerRecAttrs
-              // {
+
                 listen =
                   let
                     identity =
···
                         key-file = "${certs.${names.cert}.directory}/key.pem";
                         certificate-file = "${certs.${names.cert}.directory}/fullchain.pem";
                       };
+
+                    baseListen =
+                      {
+                        port = port.TLS;
+                        ssl = (lib.recursiveUpdate tlsRecAttrs value.tls.extraSettings) // {
+                          inherit identity;
+                        };
+                      }
+                      // lib.optionalAttrs (value.host != null) {
+                        host = value.host;
+                      };
+
+                    # QUIC, if used, will duplicate the TLS over TCP directive, but
+                    # append some extra QUIC-related settings
+                    quicListen = lib.optional (value.tls.quic != null) (baseListen // { inherit (value.tls) quic; });
                   in
                   {
-                    port = port.TLS;
-                    ssl = (lib.recursiveUpdate tlsRecAttrs value.tls.extraSettings) // {
-                      inherit identity;
-                    };
-                  }
-                  // lib.optionalAttrs (value.host != null) {
-                    host = value.host;
+                    listen = [ baseListen ] ++ quicListen;
                   };
-              };
+              in
+              value.settings // headerRecAttrs // listen;
           };
     in
     # With a high likelihood of HTTP & ACME challenges being on the same port,

nixos/modules/services/web-servers/h2o/vhost-options.nix

···
                   '';
             };
             recommendations = tlsRecommendationsOption;
+            quic = mkOption {
+              type = types.nullOr types.attrs;
+              default = null;
+              description = ''
+                Enables HTTP/3 over QUIC on the UDP port for TLS. The attrset
+                provides fine-turning for QUIC behavior, but can be empty. See
+                <https://h2o.examp1e.net/configure/http3_directives.html#quic-attributes>.
+              '';
+              example =
+                literalExpression
+                  # nix
+                  ''
+                    {
+                      amp-limit = 2;
+                      handshake-timeout-rtt-multiplier = 300;
+                      retry = "ON";
+                    }
+                  '';
+            };
             extraSettings = mkOption {
               type = types.attrs;
               default = { };

nixos/tests/web-servers/h2o/basic.nix

···
     server =
       { pkgs, ... }:
       {
+        environment.systemPackages = [
+          pkgs.curlHTTP3
+        ];
+
         services.h2o = {
           enable = true;
           defaultHTTPListenPort = port.HTTP;
···
             "${domain.TLS}" = {
               tls = {
                 policy = "force";
+                quic = {
+                  retry = "ON";
+                };
                 identity = [
                   {
                     key-file = ../../common/acme/server/acme.test.key.pem;
···
         ];
 
         networking = {
-          firewall.allowedTCPPorts = with port; [
-            HTTP
-            TLS
-          ];
+          firewall = {
+            allowedTCPPorts = with port; [
+              HTTP
+              TLS
+            ];
+            allowedUDPPorts = with port; [
+              TLS
+            ];
+          };
           extraHosts = ''
             127.0.0.1 ${domain.HTTP}
             127.0.0.1 ${domain.TLS}
···
       assert "content-type: text/x-rst" in tls_hello_world_head
 
       assert "${sawatdi_chao_lok}" in server.succeed("curl -v --http2 --tlsv1.3 --compressed --fail-with-body 'https://${domain.TLS}:${portStrTLS}/hello_world.rst'")
+
+      quic_hello_world_head = server.succeed("curl -v --head --compressed --http3-only --fail-with-body 'https://${domain.TLS}:${portStrTLS}/hello_world.rst'").lower()
+      assert "http/3 200" in quic_hello_world_head
+      assert "server: h2o" in quic_hello_world_head
+      assert "content-type: text/x-rst" in quic_hello_world_head
+
+      assert "${sawatdi_chao_lok}" in server.succeed("curl -v --http3-only --compressed --fail-with-body 'https://${domain.TLS}:${portStrTLS}/hello_world.rst'")
 
       assert "redirected" in server.succeed("curl -v --head --fail-with-body 'http://${domain.TLS}:${portStrHTTP}/hello_world.rst'").lower()